DESCRIPTION The reticulospinal system (RS) is a major descending motor control system. It has not been possible to specifically stimulate or lesion discrete functional subsets of RS neurons and so our understanding of RS signals being transmitted to the spinal cord is poorly understood. The transparent hindbrain of the larval Zebrafish which is accessible to optical techniques solves this problem. Further, the larvae's RS system is quite simple. It is comprised of about 100 neurons, most of which can be identified in the living fish. This allows both optical recording of neural activity and laser ablation of either individual neurons or subsets of neurons. Further, given the small total number of neurons, it is feasible to make precise ablations and quantify the effects on such larval behavior as swimming, escaping, feeding and more complex behaviors. Thus, the larval Zebrafish hindbrain provides an ideal opportunity for understanding the neural control of different behaviors. The first aim is to ablate all RS neurons to delimit the range of behaviors that the RS system mediates. The second aim will use optical recording and laser ablation to identify which specific RS neurons are involved in which locomotor behaviors. This will not only reveal the functional organization of the Zebrafish hindbrain but allow the evaluation of several competing hypotheses on the forms of network organization used to implement motor control.